1. Field of Invention
This invention relates to the field of optical interferometric sensors such as Mach-Zehnder and Michelson interferometers used in optical hydrophones using a frequency modulated laser light source and more particularly to the field of apparatus and circuits for reducing or eliminating the effects of unwanted laser intensity modulation that results from laser frequency modulation.
2. Description of Related Art
This invention relates to apparatus and circuits used in the field of interferometric sensors that respond to perturbations such as acoustic wavefronts by producing a phase difference in two light beams propagated by fiber optic material. More particularly, this invention relates to those apparatus and circuits used to recover the acoustic input signal from the signal developed by the optical fiber interferometer used to sense the acoustic energy.
A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising a first length of optical fiber and a sensing coil or arm comprising a second length of optical fiber. The sensing arm is exposed to a physical disturbance, such as an acoustical wavefront while the reference arm is isolated from the wave front.
An optical coupler divides a light signal between the arms. The signals are recombined after they have traveled the length of the reference and sensing arms at which time the light signals interfere and it becomes possible to measure the phase difference between the signals.
Authors A. Dandridge et al published a paper titled "Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier" in the IEEE Journal of Quantum Electronics, Vol QE-18, No. 10 in October 1982 that discussed a process for extracting information from the interference signal produced by a fiber optic hydrophone.
British patent 2,209,221 titled "Hydrophone Demodulator Circuit and Method" issued Oct. 23, 1991 and the innovation taught in U.S. patent application Ser. No. 07/751,358, filed Aug. 19, 1991 for an "Interferometer Phase Modulation Controller", which issued as U.S. Pat. No. 5,283,625 on Feb. 1, 1994, both to J. Bunn et al and having a common assignee, Litton Systems Inc., and which U.S. patent is incorporated herein by reference, expanded on the concepts of the Dandridge 1982 paper. These papers show the type of demodulation scheme that can be used to process the signal that is made available by the invention interferometer amplitude modulation reduction apparatus and circuit.